A radiation source (e.g., linear accelerator (LINAC)) is used in radiation treatment to apply a beam of highly energized particles (e.g., a radiation beam) to a target within a patient. A mechanical positioning system positions the radiation source (e.g., LINAC) so that the radiation beam is emitted at specific angles and distances (e.g., nodes) relative to the target. Geometric beam delivery accuracy can be improved by performing calibration and verification of the mechanical positioning system.
Calibration techniques may use both a point detector and a raster scan. In a first calibration technique, a surrogate is used for the radiation treatment beam. A point detector (e.g., photodiode) or radiation sensor (e.g., stereotactic diode detector or point scintillation detector) is placed at an isocenter of the mechanical positioning system, a surrogate (e.g., a laser beam) for the radiation beam is emitted, a raster scan of a laser beam (e.g., from a central axis laser) is performed across the point detector or radiation sensor (e.g., an initial coarse scan at 0.8 millimeter (mm) resolution over a larger region and a subsequent finer 0.4 mm resolution scan over a smaller region), and the center of the radiation beam is defined from a resulting maximum optical signal intensity of the surrogate. Axis offsets (used to position the center of the radiation beam in the correct location) are determined and stored as pointing offsets to be applied during radiation treatment.
For a point detector, such a calibration and verification method using a laser as a surrogate may take 100-200 minutes for a node-set containing 100-200 nodes and 17-33 hours for a node-set for a dynamic path involving 1000 nodes. For a radiation sensor, such a calibration and verification method takes even longer. In the above described calibration and verification method, the laser beam acts as a surrogate for the center of the radiation beam, which introduces the uncertainty of coincidence of the laser beam and treatment beam (e.g., laser-to-radiation beam coincidence) in the calibration and verification results. Further uncertainty is added by any variation in instantaneous laser intensity when the maximum optical signal intensity (e.g., peak signal) is used (e.g., laser intensity stability). Uncertainties may also be introduced into the calibration and verification due to sensitivity varying with beam angle of incidence caused by anisotropic construction of the radiation sensor (e.g., detector sensitivity variation with beam orientation).
In a second calibration technique, the radiation treatment beam is used directly. A point detector or radiation sensor is placed at an isocenter of the mechanical positioning system, a radiation beam is emitted using the LINAC, a raster scan is performed across the point detector or radiation sensor, and the center of the radiation beam is defined from a resulting maximum optical signal intensity. Axis offsets are determined and stored as pointing offsets to be applied during radiation treatment. For the second calibration technique, there are not uncertainties from a laser-to-radiation beam coincidence, but the uncertainties caused by dose-rate stability and detector sensitivity variation with beam orientation may cause the time required for calibration and verification under the second calibration technique to be greater than the time required under the first calibration technique.